Sand is a fine aggregate material that is combined with hydratable cement to make mortar, and, when crushed gravel is also included, to make concrete. Sand particles usually have a maximum size of 5 mm or less, and may be natural or manufactured.
FIG. 1 is a microphotograph of typical natural sand particles which have been weathered over many years. These sand particles may be derived from glacial, alluvial, or marine deposits, and have a generally spheroidal shape and relatively smooth surface. The shape of natural sand is much more favorable, in contrast to manufactured sand, for controlling rheology in concrete (in terms of water demand) and finishability.
FIG. 2 is a microphotograph of manufactured sand. This type of sand has a vastly different particle morphology and surface texture. The particles have angular morphologies with sharp corners and oblong and/or prolate shapes. Manufactured sand is extracted from the earth and crushed by mechanical equipment, often in multiple stages, giving rise to angular shapes.
The smooth appearance of the natural sand is markedly different from the rough, angular appearance of the manufactured sand. A pile (or plurality of) natural sand particles will tend to flow smoothly in the manner of a fluid; whereas a pile of manufactured sand tends to resist flow.
The use of manufactured sands presents three significant problems in making concrete and making structures out of concrete.
The first significant problem is that the crushing process produces an excessive amount of fines. While natural sand often contains less than 5% of material finer than 75 or 63 micron sieves, manufactured sand usually has 10% to 20% material finer than 75 or 63 microns (prior to washing, if done). Depending on the shape and particle size distribution of these fines, as well as the other ingredients in the concrete mixture, the fines may be beneficial to or harmful for concrete rheology. Further, this increase in fines can cause a decrease in bleeding, or the gradual rising of water to the surface of concrete. In hot, windy, and/or arid climates, the evaporation of water from the surface of concrete must be replaced by water migrating upward from within the concrete; otherwise plastic cracking of the concrete is likely to occur.
The second significant problem is that the manufactured sands may contain deleterious clay minerals. Clays are hydrous aluminumphyllosilicates comprised of tetrahedral and octahedral sheets. The exact natures of the layers and the cations between the layers determine the behavior of the clay. Expansive clays contain exchangeable cations between the layers that can be hydrated, resulting in increased spacing between layers (swelling). In contrast, the layers in non-expansive clay—such as illite, mica, and kaolin—are held closely together. Clays exhibit surface charges and have very fine particle size (typically less than 2 microns). Both expansive and non-expansive clays negatively impact concrete behavior by increasing the amount of water needed for achieving a desired concrete rheology. The effect of non-expansive clays is due mostly to the small particle size, surface charge, and poor particle shape. For instance, mica has a flat, flakey particle shape and can break down upon shear (such as during concrete mixing), resulting in very poor concrete workability. It is believed that expansive clays have a greater influence on concrete rheology than non-expansive clays because they can expand and consume free water from the concrete mixture. In addition, expansive clays are known to impede the performance of polycarboxylate type superplasticizers. Such polycarboxylate type super-plasticizers are intended to adsorb onto cement particles and to disperse them within an aqueous slurry or paste. Expansive clays interfere with this function and, for the most part, require larger amounts of superplasticizers to be used for attaining a given level of workability in plastic concrete.
The third significant problem is that the washing of excessive fines and clays from sand introduces not only the issue of added costs and disposal, but also gives rise to potential negative effects in concrete or mortar. If the amount of washing is inadequate, some clay will be invariably left in the sand and affect the behavior of polycarboxylate dispersants; but if too much of the fines are washed out, this could result in a deficiency in fines (a certain minimum amount of which is beneficial) which, in turn, adversely affects the rheology of the concrete. Moreover, washing fines does not avoid the above rheology and finishability disadvantages of manufactured sands.
While the foregoing significant problems of manufactured sand may be somewhat rectified by increasing the water or chemical admixtures used in the concrete, these methods could create additional problems. An increase in water content (to improve workability) tends to reduce strength and durability of the concrete. Increasing the amount of cement and/or chemical admixtures could offset this, but this would increase costs without resolving bleeding and finishability problems caused by the use of manufactured sand.
Technologies are available for detecting the level and mitigating the effects of clay contained in the sand aggregates used for making concrete. However, these technologies do not resolve the significant problems created by the use of manufactured sand as described above.
A novel and inventive composition and method are thus needed for treatment of clay-bearing manufactured sand which is intended for use in hydratable cementitious compositions such as concrete. The present invention provides a composition and method for mitigating clay in terms of minimizing its deleterious effects on concrete workability and/or on dosage efficiency of polymer dispersants used in the concrete.